This invention relates to a liquid quantity sensor of the type of detecting the quantity of liquid in a vessel by responding to a variation in the capacitance between two metal plates vertically disposed in the vessel and a method of producing the same. For example, the vessel is a fuel tank in a vehicle.
It is possible to detect the quantity of a liquid in a vessel by fixedly and vertically disposing two adequately spaced flat and parallel, or cylindrical and concentric electrode plates in the vessel and measuring the capacitance between these two electrode plates. This method is based on the fact that most liquid materials have dielectric constants considerably different from the dielectric constant of air. As the liquid level in the vessel rises or lowers, the submerged fraction of the effective surface area of the capacitor formed by the two electrode plates increases or decreases with a corresponding change in the value of capacitance between the two electrode plates.
When a vessel provided with a liquid quantity sensor of the capacitor type is stationary there is little problem in accurately detecting the quantity of a liquid contained in the vessel. However, the situation is different when the vessel is installed on a vehicle such as an automobile as a fuel tank. In this case difficulties arise in accurately detecting the quantity of fuel in the tank while the vehicle runs because the fuel tank becomes inclined to various degrees, causing the fuel to sway and undulate. To resolve such difficulties it is usual to use a liquid quantity sensor having plural parallel connected sets of capacitor electrode plates which are disposed at suitably selected locations in the fuel tank. Irrespective of the configuration and arrangement of the electrode plates, it is very important for accurate detection of the liquid quantity to keep an accurate and uniform distance between each pair of electrode plates. In this regard, British patent application publication No. 2,089,516A shows a liquid quantity sensor of the capacitor type in which a plurality of insulating spacers all of which have an accurately determined length or thickness are inserted between each pair of electrode plates.
However, known liquid quantity sensors of the capacitor type are not yet fully satisfactory in the accuracy of measurements when used, for example, in fuel tanks of automobiles.